You say essential, so I would suggest Revan, simply because it answers important questions and establishes the basis of SWTOR, especially the JK story. I would also suggest Annihilation because unlike other TOR novels it doesn't give you flimsy one off characters with no real depth who are only really there for the main character e.g. Malgus to bounce off from. The characters are either well fleshed out or already present it SWTOR lore. It also covers some pretty important events that occur soon after the end of Act 3. Basically these two stories complete the current SWTOR tale.

However I have to say they are far from the best. They're good but if you really just want to read good Star Wars fiction I'd highly recommend the Darth Bane Trilogy, Darth Plagueis and Labyrinth of Evil. The first two especially if you prefer the villains to the heroes. And Darth Bane even more especially because it's just plain awesome.